Spirobifluorene‐Based o‐Carboranyl Compounds: Insights into the Rotational Effect of Carborane Cages on Photoluminescence

Abstract

9,9′‐Spirobifluorene‐based closo‐o‐carboranyl (SFC1 and SFC2) compounds and their nido‐derivatives (nido‐SFC1 and nido‐SFC2) were prepared and characterized. The two closo‐compounds displayed major absorption bands assignable to π–π* transitions involving the spirobifluorene group, as well as weak intramolecular charge‐transfer (ICT) transitions between the o‐carboranes and their spirobifluorene moieties. The nido‐compounds exhibited slightly blueshifted absorption bands resulting from the absence of the ICT transitions corresponding to the o‐carborane moieties due to the anionic character of the nido‐o‐carboranes. While SFC1 exhibited only high‐energy emissions in THF at 298 K (only from locally excited (LE) states assignable to π–π* transitions on the spirobifluorene group), remarkable emissions in the low‐energy region were observed in the rigid state such as in THF at 77 K and in the film state. SFC2 displayed intense emissions in the low‐energy region in all states. The fact that neither of the nido‐derivatives of SFC1 and SFC2 exhibited low‐energy emissions and the TD‐DFT calculation results of each closo‐compound clearly verified that the low‐energy emission was based on ICT‐based radiative decay. The conformational barriers from each relative energy calculation upon changing the dihedral angles around the o‐carborane cages for both compounds confirmed that the rotation of the o‐carborane cages and terminal phenyl rings for SFC1 is freer than that for SFC2.

Boosting Circularly Polarized Luminescence of Organic Conjugated Systems via Twisted Intramolecular Charge Transfer

“…As f w was 95%, the λ em value of o-1 had an obvious blue shift by 31 nm with respect to f w of 90%, while o-2 was only slightly blue shifted by 9 nm [28]. It was clearly demonstrated that the red-shifted spectra were attributed to the lowest TICT excited states [30]. On the other hand, the selective introduction of ocarborane units onto the BINOL skeletons also exerted influence on their aggregated patterns, resulting in different aggregation-induced emission-(AIE-) active properties.…”

Recent Progress in the Development of Solid‐State Luminescent o‐Carboranes with Stimuli Responsivity

“…The planar conformation of the conjugated system and perpendicular orientation of the C−C bond in the o ‐carborane and π‐planes are favorable for presenting AIE with ICT character. Moreover, AIE efficiency was examined with spirobifluorene‐tethered o ‐carboranes and nido ‐carboranes ( 38 – 41 ) . It was mentioned that the perpendicular orientation is also advantageous for obtaining higher efficiency of AIE.…”

Strategic molecular design of closo-ortho-carboranyl luminophores to manifest thermally activated delayed fluorescence

“…Fox et al showed that the D-A dyad, C-diazaboryl-o-carborane, exhibits two different excited states (locally excited (LE) and ICT transition states), depending on the rotational motion of diazaboryl derivatives; 26 this phenomenon is called the twisted intramolecular charge-transfer (TICT) effect. [46][47][48][49][50][51][52][53][54][55][56] The occupation of the lowest unoccupied molecular orbital (LUMO) levels for an o-carborane cage is dramatically altered from 36% to 98% in accordance with the motion. This indicates that the electronic separation between the donor and acceptor moieties is governed by structural motion.…”